A new Compton x-ray backscatter imaging (CBI) technique, backscatter radiography by selective detection (RSD), has been successfully employed in the non-destructive evaluation (NDE) of a wide variety of interrogation applications. This enhanced CBI technique, and the various RSD modalities being used today, has been under development since 1986 when it was first used for detection of buried land mines.

Prototype landmine scanning system

        Front Collimated     Uncollimated     Back Collimated    Final Image

Radiography by selective detection allows for preferential detection of backscatter components that are responsible for improving image contrast. This is accomplished through a set of specially designed detectors and both fixed and movable detector collimators. An example of RSD is lateral migration radiography (LMR). The LMR approach is based on image contrast generated by the lateral migration of scattered x-rays in directions perpendicular to the radiation illumination beam. This technique has been used successfully to detect buried landmines and flaws and defects in aluminum, plastics, honeycomb laminates, reinforced carbon composites, steel, and titanium. LMR has been used to detect variations in composition, including corrosion, and to detect flaws as small as 50 microns in height, 1 mm in width and 1 cm long in samples of actual aircraft components.

RSD scan of objects through 1.5” of gypsum

In addition to finding defects in materials, RSD has demonstrated the ability to image the inside of gypsum walls, concrete block, and probe several centimeters of solid concrete. The backscatter images have high enough contrast and resolution to identify wires and fiber optics imbedded in walls and even the individual electronic components of a small radio through 3.8 cm of gypsum. RSD has also demonstrated the ability to detect boric acid deposits, and defects in steel through 7.6 cm of the stainless-steel reflective insulation used in commercial nuclear reactor applications. This allows for the remote inspection of nuclear reactor pressure vessel components for boric acid deposits and defects without having to remove the insulation.

Lockheed Martin Space Co. and NASA are currently using five x-ray backscatter RSD scanning systems developed by the University of Florida to inspect the spray-on foam insulation (SOFI) used on the external tank of the space shuttle for the launch scheduled for May-June, 2005. This RSD system can detect the type of flaws (voids and delaminations) that were responsible for insulation striking and damaging the leading edge of the wing of the Columbia Space Shuttle during the last launch (STS-107). The inspection of the SOFI will reduce the risk of this type of accident occurring in the future.

RSD image of a dime (top), 50 mg (left), and 100 mg (right) of boric acid residue on steel
through TRANSCO insulation

X-ray backscatter RSD is a proven technology that has been tested on a wide variety of materials and applications. The focus of RSD has been one-sided detection for applications where conventional non-destructive examination methods either will not work or give poor results. Acquired images have clearly shown, for a variety of conditions, that proper selection of x-ray field scatter components leads to a significant improvement in image quality and contrast. Improvements are significant enough in some cases that objects not visible to conventional CBI or transmission radiography become readily discernable with LMR or RSD.

Simplified schematic of a RSD subsurface-feature experimental setup with a collimated and uncollimated detector